The present invention relates to an extensometer that provides more than one gage length in a simple, reliable, and very accurate attachment sequence.
In the prior art, precision extensometers for determining the strain in specimens have been well known. For example, U.S. Pat. No. 3,789,508 illustrates a typical extensometer of the type disclosed herein. U.S. Pat. No. 4,507,871 shows low mass reliable attachment springs for attaching the extensometers to a specimen. MTS Systems Corporation, of Minneapolis Minnesota, the assignee of the present application, has sold gage length extenders for extensometers, but in systems wherein each individual extended gage length requires an entirely separate extender that is fastened on to the extensometer itself. The extensometer extenders such as the MTS Model Series 623.15 include separate members utilizing a base plate member and a top or head member which add mass, and also are relatively expensive to make.
Gage length de-extenders (shorteners) have been added to extensometers such as those shown in U.S. Pat. No. 3,789,508 to make the gage length shorter. Adapters are added to the extensometer arms between the specimen and the arm ends and thus the extensometer is moved farther from the central axis of the specimen than without the adapters. An undesirable moment is produced on the specimen and extensometer. The adapters also add mass. Added mass, and/or moments are undesirable when making dynamic strain measurements.
The spring mountings or attachment devices shown in U.S. Pat. No. 4,507,871, are desirable because they are light weight and provide a force that urges the specimen engagement knife edges against the specimen, with such force directly in alignment with the knife edges to prevent unwanted moments. With the deextenders used in the prior art, the spring attachments no longer were in line with the knife edge which produces an undesirable moment on the extensometer.